[mapserver-users] WMS layer filtering

[Julien Cigar]

On 06/12/2013 14:23, Kralidis,Tom [Ontario] wrote:
> Hi: we have a MapServer OWS instance which we use to disseminate
> observations via WMS and WFS/SOS.
>
> One of our layers has 1 million records (PostGIS backend).  The layer
> has CLASS level EXPRESSION objects for WMS visualization/classification.
> We initially implemented a LAYER level FILTER, however we have use cases
> to serve the same layer (with the same identifier) via WFS, in which
> case we'd like all the data to be available.
>
> Our problem happens when visualizing via WMS.  We have a Boolean column
> in our PostGIS table ('latest_observation') which allows us to filter
> easily to visualize latest observations, which is our desired WMS
> default.
>
> Because we have defined CLASS level filters, the WMS GetMap request is
> first querying and fetching all records and _then_ applying CLASS level
> filters (which all include 'latest_observation=1'.  So this ends up
> hurting us performance wise.
>
> We'd like to have this filter at the query level so that less members
> are passed back from the SQL query.
>
> When we do a LAYER.FILTER with latest_observation=1, the result is
> instant (beautiful!).  But if we put a LAYER.FILTER, this constrains the
> data access from the WFS perspective against the same LAYER def.
>
> Is there any way to apply a LAYER.FILTER type approach to work _just_ in
> WMS mode? We'd like all the data in WFS mode.
>
> I know we can duplicate the LAYER object as a workaround, but from a
> data management perspective we'd rather now.
>
> I hope I have explained this clearly enough.  Any suggestions?

It may seems strange (?), but there is nothing in the WMS spec to filter 
a WMS layer (that's why Geoserver has CQL filters), and I think the "OGC 
way" to filter a WMS request is through an SLD stylesheet (which can be 
generated dynamically).

> Thanks
>
> ..Tom
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